1. Field of the Invention
The present invention relates generally to oxygen sensors. More particularly, the present invention relates to an adjustment circuit for adjusting a measurement output of an automotive oxygen sensor.
2. Discussion of the Related Art
In the automotive industry, many design issues such as engine efficiency and emissions control present substantial challenges to scientists and engineers. A particular parameter of interest relating to the above design issues is the oxygen (O2) level of the automotive exhaust. It is therefore common to install one or more oxygen sensors into the manifold of vehicles at locations where exhaust from all cylinders has merged in an effort to monitor the oxygen level of the exhaust. The resulting analog signal from each sensor corresponds to a detected oxygen level and is typically fed to an A/D converter, and then to an engine controller for processing. The A/D converter transforms the analog signal into a digital value and the engine controller uses the digital value to perform many functions throughout the vehicle. Under normal operating conditions, the result is a closed loop control system for maintaining a desired engine efficiency and oxygen level in the exhaust.
It is well known that modern day oxygen sensors have a measurement output and a sensor return. A typical oxygen sensor will have a measurement output range of 0 to 1 volts relative to the sensor return. Conventional circuits connect the sensor return to ground, and apply the measurement output directly to the A/D converter. A difficulty associated with this approach, however, relates to the fact that oxygen sensors have the tendency to invert when the temperature of the sensor reaches a given temperature threshold. It can be shown that the inversion is typically due to outgassing. Thus, at very hot temperatures the measurement output will invert, resulting in a voltage between 0 and xe2x88x921 volts relative to the sensor return. It is important to note, however, that the absolute value of the measurement output is still accurate. Nevertheless, the effective sensor voltage range is xe2x88x921 to +1 volts.
The above inversion phenomenon causes a number of operational difficulties. For example, the typical embedded controller will have an A/D converter with an input range of 0 to 5 volts, thereby representing only positive voltages. Thus, when the measurement output inverts, the operation range of the converter is breached and the engine controller will essentially ignore the output of the A/D converter. The result is an open loop control system with respect to automotive exhaust oxygen levels. The open loop system causes poor engine efficiency and emissions control. It is therefore desirable to provide an adjustment circuit and method for adjusting a measurement output of an automotive oxygen sensor such that inversion of the measurement output does not result in open loop control.
It is also important to note that since the A/D converter has a range of 0 to 5 volts as opposed to the 0 to 1 volt range of the measurement output, the A/D converter""s effective resolution is reduced by 80%. Furthermore, the relatively small measurement output of the sensor causes the signal-to-noise ratio (SNR) to become a very important issue. In order to improve the accuracy of the overall system, conventional approaches involve dedicating a separate ground reference to the sensor return. It is therefore desirable to provide an approach to maximizing the operational range of the A/D converter in view of the significantly narrower sensor voltage range.
The above and other objectives are provided by an adjustment circuit and method in accordance with the present invention for reading a measurement output of an automotive oxygen sensor. The adjustment circuit includes a biasing stage connected to a sensor return of the oxygen sensor, where the biasing stage applies a predetermined bias voltage to the sensor return. An input stage is connected to an output of the sensor for retrieving the measurement output from the sensor. The adjustment circuit further includes an A/D conversion system connected to the input stage for adjusting the measurement output based on the bias voltage. The A/D conversion system may further be connected to the biasing stage for retrieving a sensor return output from the sensor. The sensor return output defines an actual bias voltage applied to the sensor return. In such cases, a differential module calculates a difference between the sensor return output and the measurement output.
The present invention also provides a method for reading a measurement output of an automotive oxygen sensor, where the oxygen sensor has a sensor return. The method includes the steps of applying a predetermined bias voltage to the sensor return, and retrieving the measurement output from sensor. The measurement output is then adjusted based on the bias voltage.
Further in accordance with the present invention, a method for adjusting an oxygen sensor measurement output based on a predetermined bias voltage applied to a sensor return is provided. The method includes the step of determining an adjusted output based on the bias voltage and the measurement output. An absolute value of the adjusted output is then calculated, where the absolute value corresponds to a detected oxygen level. The method further includes the step of generating a digital value based on the absolute value, where the digital value corresponds to a detected oxygen level. Use of a bias voltage allows correction for outgassing of the oxygen sensor. The result is a more effective engine control loop with respect to oxygen levels.
Further objectives, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.